Further examination of N- and B-tropic MSV pseudotypes from clones of C3H cells transformed by Ki-MSV(B) and 3T3 cells transformed by S plus L minus MSV(N) is proposed order to know by which mechanism(s) in a nonpermissive system some cells are permissive to virus replication. Focus forming assays on N-type cells and B-type cells systems with N- and B-tropic S plus L minus MSV pseudotypes and N, B, and NB helper virus will be performed in order to determine the nature of virus-virus interactions in these systems after dual infection.